Over the years, in situ spectroscopic studies have contributed largely to an in-depth mechanistic understanding of many homogeneously catalyzed reactions. This review discusses the application of operando spectroscopy such as high pressure infrared and NMR to obtain in-depth mechanistic understanding of transition metal catalyzed carbonylation reactions. Several catalytic reactions of industrial relevance are discussed including rhodium catalyzed hydroformylation, palladium catalyzed alkoxycarbonylation, and copolymerization of CO and alkenes. Also, mechanistic studies of tandem reactions such as rhodium catalyzed hydroaminomethylation and hydroformylation-hydrogenation are included. In addition, the effects of alternative media such as supercritical CO₂ and ionic liquids are discussed.